Four-wheel drive motor vehicles are becoming increasingly popular. Recently, certain motor vehicles have been provided with "full-time" four-wheel drive systems also known as all-wheel drive systems. In such all-wheel drive systems, the torque transfer cases are typically provided with an interaxle differential for dividing torque between the front and rear wheels of the motor vehicle. The interaxle differential enables the front wheels and the rear wheels to rotate at different speeds, during normal turning of the motor vehicle or in the event that the front wheels and the rear wheels have tires with different diameters. However, to prevent excessive relative slipping between the front wheels and the rear wheels, as might occur when one set of wheels encounters a low-traction condition, such as ice, these transfer cases typically include a selectively engageable clutch which is operative to lock the interaxle differential upon sensing a predetermined amount of relative slippage between the front output shaft and the rear output shaft of the transfer case. Locking of the interaxle differential prevents any further relative overrun or differentiation between the front output shaft and the rear output shaft of the transfer case.
Known prior all-wheel drive systems have generally required complex electronic sensors or other complex systems to monitor the overrun or differentiation between the transfer case front and rear output shafts or the front wheels and the rear wheels of a motor vehicle. Upon sensing relative overrun or differentiation, an electronic control system determines whether the relative overrun or differentiation being encountered is within a "normal" expected range or is "excessive." If the electronic control system indicates that the overrun or differentiation being experienced is "excessive," the electronic control system causes the clutch to lock the interaxle differential to preclude any further relative overrun or differentiation. An electronic control system of this type can be expensive to manufacture and maintain and a more cost-effective, simplified "on demand" system of limiting more than a predetermined amount of overrun or differentiation between the front wheels and the rear wheels of the motor vehicle would be desirable.
In recent years, motor vehicle all-wheel drive power-train systems have been cost-reduced by eliminating the transfer case differential, providing continuous power to a primary axle, and providing on-demand power to the secondary axle whenever primary axle slippage occurs. Typically, a torque-coupling device (viscous, hydraulic, electric) is utilized within the transfer case to drive the secondary axle. The torque-coupling device compensates for any speed difference which may occur between the primary and secondary axle. When the secondary axle is a front axle, an open differential is typically installed. Torque-sensing differentials with clutch pack pre-load are not considered acceptable for front axles due to steering issues attributable to torque bias. The front wheels tend to slide rather than turn on low coefficient-of-friction surfaces.
One prior art vehicle is offered with a speed-sensing, limited slip front axle differential to eliminate the pre-load torque bias issue. However, this design still requires an interaxle differential or in-line torque coupling to avoid driveline wind-up when operated on hard pavement.
It would therefore be desirable to provide a simplified, cost effective mechanism to allow differentiation between front and rear output-shafts in an all-wheel drive system, while providing speed-sensing limited-slip capabilities.